The present invention generally relates to α-sulfofatty acid ester compositions and methods for making and using such compositions. More particularly, the present invention relates to α-sulfofatty acid ester compositions that are enriched for particular α-sulfofatty acid esters, and methods for making and using such compositions.
Detergents have been used for many years to clean clothing and other materials. Detergent compositions are generally formulated to contain components such as surfactants, builders, electrolytes and other additives dispersed or dissolved in an aqueous medium. Surfactants are included in detergents to enhance their cleaning performance. Typical surfactants include anionics, nonionics, zwitterionics, ampholytics, cationics and those described in Surface Active Agents, Volumes I and II by Schwartz, Perry and Berch (New York, Interscience Publishers), Nonionic Surfactants ed. by M. J. Schick (New York, M. Dekker, 1967), and in McCutcheon's Emulsifiers & Detergents (1989 Annual, M. C. Publishing Co.), the disclosures of which are incorporated herein by reference.
Recently, interest in α-sulfofatty acid esters (also referred to hereinafter as “sulfofatty acids”) has increased due to the superior cleaning properties of these surfactants in hard water. While other surfactants have similar detergency in soft water, as water hardness increases α-sulfofatty acid esters exhibit increased cleaning performance as compared with other anionic surfactants. Thus, α-sulfofatty acid esters are an effective anionic surfactant that can be used in a wide variety of washing conditions.
α-Sulfofatty acid esters are typically manufactured as salts (i.e., a mixture of salt forms, typically mono- and di-salts). Di-salts result from hydrolysis of α-sulfofatty acid ester during manufacture, storage and in detergent compositions. In particular, mono-salts of α-sulfofatty acid ester hydrolyze in the presence of moisture and alkali-containing detergent components to form di-salts. For example, in formulations containing methyl ester sulfonate (“MES”) that is well mixed with high pH components under aqueous conditions, the MES will hydrolyze nearly completely to the di-salt form. Such high pH components can include builders, such as silicates or carbonates, and bases. This chemical instability discourages the use of such anionic surfactants in many applications.
α-Sulfofatty acid esters are typically made from natural fats or oils. These fats and oils are usually free fatty acids or glycerol esters (i.e., mono-, di- or triglycerides). Free fatty acids are formed from glycerol esters by hydrolysis. The resulting fatty acids typically contain an even number of carbon atoms. These fatty acids are then esterified to form fatty acid esters. The esters are then sulfonated to form α-sulfofatty acid esters.
The surface active agent properties of α-sulfofatty acid esters are related to the chain lengths of the fatty acid portion of the molecules. For example, shorter chain length molecules (e.g., C8-C12 α-sulfofatty acid esters) are typically more water-soluble, but exhibit lesser surface active agent properties. Longer chain length (e.g., C14-C16) α-sulfofatty acid esters exhibit greater surface active agent properties, but are less water-soluble. Because α-sulfofatty acid esters are usually manufactured from natural sources, they are a mixture of different chain lengths. The properties of such mixtures are determined by the chain length distribution of the source of the fatty acids. Thus, α-sulfofatty acid esters from palm kernel (stearin) oil have different surfactant properties than α-sulfofatty acid esters from tallow. To overcome the limitations of single sources, blends of α-sulfofatty acid esters from multiple sources are prepared. Such blends are also limited, however, by the chain length distributions of each fatty acid source. For example, combining α-sulfofatty acid esters from palm kernel (stearin) oil (C6-C20 fatty acids) with α-sulfofatty acid esters from tallow (C14-C18 fatty acids) creates a mixture that has the average characteristics of the α-sulfofatty acid esters contained therein.
It has not been previously appreciated that the properties of α-sulfofatty acid ester mixtures can be improved by enriching such mixtures with particular chain length α-sulfofatty acid esters. Such enrichment allows certain beneficial properties (associated with particular chain length α-sulfofatty acid ester) to be enhanced without diluting such mixtures with other α-sulfofatty acid ester chain lengths.